A review on apomeiosis in Poa pratensis and Medicago sativa mutants.

In contrast to sexual reproduction, apomixis bypasses meiosis and egg cell fertilization. Gametophytic apomixis occurs with the parthenogenetic development of unreduced egg cells from apomeiotic embryo sacs originating from a nucellar somatic cell (apospory) or a megaspore mother cell with no, or modified, meiosis (diplospory). Apomeiosis, along with parthenogenesis, excludes segregation and recombination during meiosis and fertilization. Thus, understanding the genetic control and the molecular mechanisms underlying apomeiosis is critical for the comprehension of apomixis as a whole. In this paper we review the available data on apospory in the facultative apomictic species Kentucky bluegrass (Poa pratensis L.) and on diplospory in reproductive mutants of the sexual species alfalfa (Medicago sativa L.). Our recent acquisitions on candidate genes for apomeiosis are reported and strategies for elucidating the inheritance of this trait by means of genomics and expression studies are presented and discussed. In particular, experimental data focus on PpSERK and MsMob1 genes. We document that PpSERK transcripts are specifically expressed in the megaspore mother cells of sexual genotypes and in the aposporic initials of sexual genotypes, suggesting that PpSERK plays a role in early stages of embryo sac development. Moreover, the altered expression of MsMob1 in ovules of a reproductive mutant producing diplosporic eggs is reported, and the implication of MsMob1 proteins in programmed cell death of meiotic megaspores is also considered.